


Kinktober Day 27: Xenophilia

by tiamatv



Series: Promptober 2020 [26]
Category: Supernatural
Genre: Alternate Universe - Fantasy, Cecaelias, Comeplay, Cultural Differences, Fluff and Smut, Honeymoon, Interspecies Relationship(s), Loving Marriage, M/M, Mermaids, Oral Sex, Porn with Feelings, Prince Dean Winchester, Romantic Fluff, Size Kink, Tentacle Sex, Top Castiel/Bottom Dean Winchester, Top/Bottom Versatile Castiel/Dean Winchester, octo!cas
Language: English
Status: Completed
Published: 2020-10-28
Updated: 2020-10-28
Packaged: 2021-03-08 20:33:49
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,066
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27242749
Author URL: https://archiveofourown.org/users/tiamatv/pseuds/tiamatv
Summary: “Your mate looks very delicious,” the mermaid tells Castiel, in a weirdly deep, throaty voice. "He has beautiful spots."“Yes, he does, doesn’t he?” Castiel answers, proudly.Dean thinks—he’s pretty sure—okay, he’s almost sure—that no-one’s talking about actually eating him.(No, Cas, he hasn’t forgotten the goddamned cannibal thing.)(The tentacle husbands on their honeymoon!)
Relationships: Castiel/Dean Winchester
Series: Promptober 2020 [26]
Series URL: https://archiveofourown.org/series/1954990
Comments: 39
Kudos: 201





	Kinktober Day 27: Xenophilia

**Author's Note:**

  * For [saltyravenclaw](https://archiveofourown.org/users/saltyravenclaw/gifts).



> Sooooo in what seems to be that thing that I do, I had something else planned for today, and then I was so completely inspired and thrilled by [saltyravenclaw](https://archiveofourown.org/users/saltyravenclaw/pseuds/saltyravenclaw) that my brain had a complete turnaround. (You know what you did!) So here I am writing porn in the middle of the night again.
> 
> This is mostly these two being stupidly and disgustingly in love as, y'know, husbands do on their honeymoon. There is a big dash of graphic cephaloporn. If this is not your thing, I do not blame you! There's pretty much no plot to be seen anywhere, just a prince and his cecaelia husband getting to know each other.
> 
> (Oh. But there are mermaids. Yes.)

“Your mate looks very delicious,” the mermaid tells Castiel, in a weirdly deep, throaty voice. "He has beautiful spots."

“Yes, he does, doesn’t he?” Castiel answers, proudly.

Uh, yeah, okay.

But Dean’s husband’s color play is flickering a happy gold in little twinkling stripes up and down the length of his body, catching the thin spring sunshine through the mostly calm water as his sea-arms wave gently underneath him. The sand underneath is so pale and the water so clear that it’s almost like they’re putting on a wiggly kind of show—‘see here? No threat here.’ None of Cas’s body language is stiff, his shoulders loose and relaxed as he holds onto the splash deck of Dean’s boat with one hand. The moment Dean dangled his legs into the water, earlier, Cas’s friendly sea-arm splooshed excitedly up and curled part of itself on his thighs, and some of its suckers are pulsing gently on Dean’s right calf.

So Dean thinks—he’s pretty sure—okay, he’s _almost_ sure—that no-one’s talking about actually eating him.

(No, Cas, he hasn’t forgotten the goddamned cannibal thing.)

But the mermaid studies him through big, dark eyes, and she doesn’t look grossed out or pissed-off by him the way Cas’s relatives all did. She doesn’t have much _expression,_ but at least Dean’s got a reasonable hope she’s not gonna try and rip his head off.

She introduced herself as Benjamin, and honestly? The fact that that’s a male name on land doesn’t even faze Dean, because she sure as heck doesn’t look much like Dean ever expected a mermaid to look like in the first place.

Sure, she’s a lady on top and… _sort of_ like a fish on the bottom? No fish Dean’s ever seen, though: her tail is thick and muscley, almost as long as Dean is tall, with a pair of narrow, sharp fins at the end. Two other, broader fins poke out on the underside, halfway down the length of her tail, and they seems to be wiggling to help keep her head and shoulders above water so she can talk to them. No pretty colors or bright green scales: her tail is pitch-black, darker than Cas’s hair even when it’s wet, and he can see it swishing as she levers herself a little out of the water to give him a closer look. It’s not like where Cas’s belly curves downwards and joins into his pretty blue mantle, either; in her case, her bottom half just sort of _merges_ with her top half, and Dean can’t tell where the two bits become one.

She’s got what looks a lot a belly button, though. Okay. Huh.

Benjamin has almost no hair, just a thin wet dark cap of it at the top of her head, and dark chocolate skin. Her lips are very full, and red, but when she talks, Dean can see that her teeth are pointed— _all_ of them, not just the cute little canines that Cas has on either side of his front teeth.

Dean’s also trying not to notice that she has high, small tits, but it’s not because it’s a sex thing. It's not even because of a mouth full of sharp teeth thing. And yeah, her boobs are bare—apparently mermaids don’t wear clothes underwater any more than cecaelia do. She clearly doesn’t care if anyone’s looking at them.

But they also don’t have any _nipples._

Dean has no idea why he finds _that_ , of everything that’s going on in front of him, freaky. It’s freakier than Cas’s lack of a belly button and the knowledge that he hatched out of an egg. Or, hell, Cas’s _sea-arms._ Maybe just because Dean’s gotten friendly with those, though. He glances down and pats the one that seems to like him best, the heavy cool loop of it resting on his leg. It quivers like firm, happy pudding, and the tip of it, dangling in the water along with Dean’s feet, prods his big toe like it’s asking if he’s okay.

Dean bites down a snort. His life has gotten really fucking weird.

Benjamin looks back and forth at them, and nods, small. “Because we have shared the ocean together, Castiel, and because I owe you the life of my calf,” Benjamin tells them, with that same deliberate seriousness that Cas seems to have to him a lot of the time, too, “I will do this for you.” Her lips move in something that’s probably a smile—though the skin around her eyes doesn’t move. Actually? Dean’s not sure he’s seen her _blink_. “My congratulations on your mating, Castiel of Angel Deep, Dean of the Land, and may your colors be seen for miles.”

Okay, Dean blushes maybe a little at that, because yeah, he _does_ know exactly what that means now, thanks.

He’s _trying_ to get used to the whole fucking-in-public thing, alright? Sometimes they still use the cabin bedroom, the bed _is_ easier on the knees, but the open air sex on the boat deck has been great. Like… okay, he’s gonna have sweet dreams _forever_ about Cas riding on his cock all kissed by the sunlight, his head back with his hair stiff and wrecked with drying seawater, his legs thrown open wide ‘cause Cas doesn’t bother to be graceful or neat when there’s something he likes. They haven’t tried it in the water yet—okay, not true, they did try to fool around a bit. But here in the deeper water outside of their bay, with the combination of even the small waves and the currents, even Cas couldn’t quite keep them both held up comfortably without Dean’s head going underwater.

Cas, the kinky fucker, is the one who suggested that Dean lie out on the splash deck with his legs facing the water instead. Which meant that Dean couldn’t help but see Cas’s cotylus arm wriggling its way onto the deck and inching closer and closer to him, glistening wet, those soft little sky-blue suckers pulling it along towards him. His mating colors are glowing so brightly that his whole sea-arm is practically tinted pink, and the shadow it's casting isn't just from the sun. Cas barely teases him along the creases of his groins, one side and then the other, before tucking behind his balls. And fuck, Dean's ass is already twitching with want by the time that slender tip presses into him, slipping and sliding so easy into Dean’s body ‘cause they _both_ want this again, so bad.

But they aren’t underwater, this time, and Cas’s cotylus isn’t behind Dean—it’s right between his legs. So Dean isn’t just feeling more and more of Cas’s sea-arm disappearing into him, stuffing him full as Cas moves and moans, pressing his forehead against the arms he has clutching on the edge of the deck. He’s watching it happen. They _both_ are.

And seeing all of it, _feeling_ his mate’s cotylus move and twist, the little ripples and kisses of Cas’s suckers moving in concert inside and out of him, is the most fucking impossible show. It's strange and hot, it's colorful and beautiful, and Cas is watching it happen, watching himself fuck Dean, with just as much fascination as Dean is.

Dean’s already so close to the edge by the time the smallest of the little suckers inside Dean wriggle and press and Cas’s mating ridge stretches him wide. Cas gasps, desperate and rough, and his face is so fucking _beautiful_ all flushed and lit-up in sunlight when he says Dean’s name, twice.

This position means Dean's got a perfect view _,_ this time, as the thick bulge that carries Cas’s semen travels up the length of the deep blue sea-arm buried up Dean’s ass. He can see it moving under Cas’s skin, coming closer and closer to him, and he wants to hold on to control so badly, wants to fucking just _feel_ it shove into him.

But by the time it pushes him wide, wide open, Dean’s already coming so hard that he hits himself on the chin with his own spunk. His whole body bucks so hard his ass and his back leave the boat’s surface, the cotylus just flowing and pressing and shoving into the motion the way nothing human could possibly do. He doesn’t know if that fact makes the feel of being filled up with so _much_ , the incredible stretch and then the wet gush up his ass, less intense—or more.

From the way Cas cries out so loud and deep it sounds like thunder over the water as Dean’s rim ripples around him, must be pretty nice for him, though.

So… yeah, their honeymoon? Pretty fucking awesome, so far.

When Dean pulls himself out of _that_ happy little memory, though, both his husband and Benjamin are looking at him.

From the way that they’re staring at him, at least one of them was talking to him.

Dean has no idea what they were saying.

“Well,” Benjamin says, wryly. “I see. Yes.” And, with no fanfare at all and barely a ripple in the waves as she drops underwater, she’s gone again. Dean doesn’t even see her tail flick, there’s just… nothing.

Which, considering that the sand under the boat is white, the water’s so clear that Dean can see little fish swimming at the bottom, and the scales of Benjamin’s tail are blacker than the volcanic sand in Lebanon, that is either creepy or _really_ fucking impressive.

“Oh, _shit_. Sorry. Uh, did—was that really offensive or something? That was… uh, anyway.” Dean sheepishly glances down and pets the sea-arm in his lap. It’s definitely hiding his hard-on—not that Dean thinks that Benjamin would know what that is, anyhow, but still. “Uh… what were we talking about?”

But his sweetheart’s eyes are going full-on crinkly at both corners, so blue right now they’re putting the stretch of ocean behind him to shame, and he brings one of his hands up to cover up a smile so big and bright that it makes Dean’s heart jolt. “You are making my reputation, Dean,” Cas says, and he doesn’t even pretend he’s not amused. “I am very flattered.”

Dean blinks. “What?”

“Oh. Mermaids are less visual than we are, and they rely on their other senses more,” Cas tells him, lowering his hand, and there’s a small, happy laugh that’s curving through his dark voice. “It’s very difficult to lie to them.” He’s had that little lilt to his raspy tones for days, now, and fuck, Dean thinks this might’ve been the best crazy idea he’s ever had.

Dean doesn’t remember ever being so happy. To be honest, his last experience on a boat in open water led to him being conked on the head and thrown overboard, so he wasn’t a hundred percent sure he’d be copacetic right out of the bay. (He’ll take a thousand more bonks to his very hard noggin if it’ll get him Cas all over again, but at the time it still sucked.)

But he didn’t have time to think of that with the wheel tight and steady under his hands, and beside him, Cas letting out a tiny little squeal of glee as the sail flapped open and caught the wind that guided them out of the Lebanon bay.

And out here, with the wind making wavelets that barely rock their little temporary home, the water so clear that Cas can point out a big-ass gray stingray swimming along the bottom today, or the drop-off between coral reef and the ‘deeps’ tomorrow?

Dean’s not a prince, he’s not master of the guard, he’s not Sam’s big brother or John’s eldest or the one of the Winchester boys who looks the most like Mary. A grand total of no-one gives a fuck who he is. He’s Cas’s mate, Cas is his husband, and for now, that’s all the fuck they’ve got to be.

He’s pretty sure more married people need to go on honeymoons.

Dean also honestly thinks it’s really cute that Cas, for the most part, always includes Dean in the ‘we.’ Because fuck, _yeah._

“Okay?” he agrees. “I mean, it’s not like you were planning to lie to anyone, were you?”

Cas shakes his head. “Mermaids occasionally mate and, at times, have babies with humans,” Cas tells him, seriously, and wait, _what?_ Sammy’s not going to believe any of this shit. Okay, Benjamin… _might_ know what was going on in Dean’s pants. Dean sort of fights the urge to hug Cas’s sea-arm closer to where his hard-on hasn’t died down yet, because it’s a part of Cas, not a blanket. “That’s why I thought, if anyone, one of them might be willing to help—as long as they believed us? But I don’t think Benjamin can have any doubts that I cherish you and that we belong to each other.”

Dean’s clearly missing something. “I mean, that’s good, I guess, but… why?”

“Dean, you are smelling like you want to climb into the water and mate even to _me_. Benjamin could probably smell you from a mile away,” Cas chuckles, and he chuckles louder at Dean’s muttered ‘oh, _shit._ ’ “Oh, beloved, don’t be embarrassed!” he swims his torso closer, batting those big beautiful blue eyes of his at Dean in a way that doesn’t even _remotely_ come off as innocent. “I’m _so_ pleased.”

Hell, Dean may not know too much yet about cecaelia body language, but even without the little pink grin curving his husband’s lips, he’s pretty sure that the way Cas’s sea arms are arcing gently in and out of the water and flashing a dark rose along his color stripes translates to “feelin’ smug as hell.”

He reaches out a foot and pokes Cas’s flat, pale belly with a toe. “Yeah, yeah,” he grumbles. “Laugh it up, tentacles.”

Dean probably _should_ be pissed or embarrassed or something, but one of Cas’s sea arms—not the friendly one, that one’s still curled up on Dean’s lap like a slippery cat—rears up and playfully shoves at his shoulder, knocking him back down onto his elbows with a jolt. Dean laughs “Hey!” as two of the eight flop heavily onto his chest and pin him down, one of the suckers pulling at his belly hard enough to leave a mark.

“Please, Dean. A tentacle,” Cas tells him, all playful haughty in that way that really makes Dean want to jump in the water and bite his neck, “cannot do _this._ ”

The tip of another of his sea-arms teases up the hem of Dean’s pretty much perpetually wet shorts, and slips inside.

(Yeah, it’s been days since Dean bothered with underwear. What’s the point?)

Another thing they’ve discovered? The feel of Cas’s suckers on Dean’s dick is _awesome,_ like a dozen sucking kisses all at once, right next to each other on his skin _._ Cas doesn’t even have to move them, just wrap and _hold,_ with the way the tiny little flexible muscles just brush and stroke along Dean’s cock in the most perfect mix of tease and tug. _Especially_ when one of the bigger cups molds itself right over the whole tip of Dean’s cock, sucking on him right as he’s about to come, and—mm.

Oh, fuck yes.

Dean rolls his hips into the easy, lazy, jerky pleasure of it. Another sea-arm strokes and nuzzles the inside of his right thigh in little pulses as Dean makes a mess of his shorts—again.

_Mmm!_

Yeah, he’s not exactly sure why he still bothers to wear shorts, either.

Dean lazily pushes himself back into a sitting position when he’s all done, his skin hot and sticky with drying seawater and sweat. Before the arm that’s been so nice to him escapes back into the water, though, he reaches for it, grabbing playfully at the muscled core. It jerks straight in the air, tip pointing up into the sky like it’s surprised, and Dean laughs.

“What?” he tells Cas—or maybe it, he’s still not quite sure. “I’m not rude, you gotta let me return the favor.”

“I’m so delighted that you enjoy my suckers, Dean,” Cas murmurs, as Dean brings the sea arm to his face and presses a kiss to the little curl at the very end. This was weird to him, in the beginning, but the more he thinks about it as like sucking on Cas’s fingers, only better, the more he’s gotten to like it. A tiny sucker kisses his cheekbone as he makes his way along them, moving to the bigger and bigger ones. He watches as Cas goes lazy and soft, moaning quietly and flopping his torso facedown on the edge of the deck, but there’s nothing impatient about it. Dean tastes both of their salt, cleaning his come up in lazy slurps from around the soft, rippling little pastel pods on the underside of Cas’s sea-arm.

Cas can’t exactly come in the same sort of ridiculously intense, one-shot way he does from his cotylus from someone playing with his suckers, but they _are_ really sensitive. (Also, Cas having that kind of orgasm normally wipes him out for _hours_. It’s the most adorable thing—he ends up sort of floating around with all of his sea-arms completely limp, just his eyes and sometimes the tip of his nose barely peeking out of the water, bits of white light leaking out from under his half-closed eyelids. Dean’s never seen anyone looking so completely blissed. He was even worried for a second that Cas might slip under the water or something, before he realized just how _stupid_ a thought that was.)

Cas seems to get another, different kind of pleasure from his suckers being played with, though, something small and slow and ripply where his cotylus gets hard but just sort of _dribbles_ rather than, well, exploding. And he whines so goddamned _prettily_ when Dean nibbles on them.

It doesn’t take more than a few minutes before Cas is there, too. He makes a sort of low, lazy purr sound that trails off in a long, happy whine of _“Deaaaan_.” His shoulders quiver, bowing inwards. Dean reaches out a hand to stroke Cas’s hair as he watches his husband come, his whole mantle shuddering and shivering in a delicate wave.

Dean’s not gonna say he’s not interested in Cas’s caves, or his home, or to swim through an undersea wreck—yeah, Sam’s really jealous about that one. He’s never seen real coral rather than just its bones (he knew it grew underwater, but he didn’t know it was _alive,_ he thought it was a kind of rock, what the fuck) and he’ll admit that he’s now really curious about puffer fish (Cas apparently loves them, though Dean hasn’t quite figured out yet why he calls them the bees of the ocean). Or even just _swimming_ with him, deep and fast, ‘cause Dean’s a strong swimmer and getting stronger every day, but every time Cas zips off to hunt, Dean realizes with a pang just how much he’s holding his husband back, out here.

(The little slices of raw fish, seasoned with seawater and wrapped with some kind of bubbly-looking and surprisingly spicy seaweed, are better than Dean thought they would be. A _lot_ better. Yeah, Dean promised he’d try everything once, and he still can’t say ‘no’ when Cas looks so proud of himself for bringing him back a treat. And, honestly? He’d definitely eat any of that again. But Cas still pouts when Dean says, “Still think I prefer it cooked, sorry, buddy.”)

But the hungry, happy, possessive look that takes over Cas’s eyes when Dean licks and sucks his own come out from around his suckers, the edges of the little disks rippling with delight against his lips, the way Cas can have two or three long slow orgasms in a row in his true form… yeah, that’s _really_ making Dean hopeful for whatever it is Benjamin’s gonna do to help them out.

They slip off the splash deck and into the water together once Dean’s pretty sure his legs will hold him again. Dean narrows his body and points his toes, lets himself sink deep enough into the sun-warm ocean to get his hair wet before he kicks back to the surface. He chuckles as one of Cas’s sea-arms wafts over to greet him again, attaching suckers carefully to his hip because Cas really, _really_ likes to touch him when they’re in the water. It flashes soft sunshine yellow in a broad stripe.

Those big sea-arms are always so gentle with him, though. It’s really sweet. Well, unless Cas doesn’t mean them to be gentle. Dean knows just how strong they are: two of them can lift _Dean._

Or throw him. All the way out of the water and back onto the boat.

“Remember when you thought I was drowning the other day?” he chuckles, paddling to keep his head above water as another sea-arm wiggles over to twine around his shoulders, and the one that was attached to him lets go. Kind of grudgingly, almost. Dean sometimes has the weird feeling that he’s married to more than one being.

Cas looks embarrassed, his eyes ducking low and sweet to be shaded by his clumped, wet eyelashes. He bobs lower in the water. “ _I_ didn’t,” he complains. “I know you can swim, Dean! You just, you fell! You hit the water very awkwardly and I saw the bubbles rush out of you! It was a reflex, my arms got worried.”

“Uh-huh,” Dean answers, his tongue in his cheek. “You know, when we get this spell thing going, I’m going to find out just how much of this stuff about your sea-arms you’re just making up, right?”

Cas’s loud _“Hmph_ ” is so completely human that he’s only got himself to blame for Dean pulling him in by the back of the neck for a wet, salty kiss.

Not that Cas seems to have any problem with that, either.

Cas changes back into his two-legger form to join Dean for dinner—well, Dean’s having dinner, and yes, it’s cooked; he knows Cas already ate. Cas doesn’t bother to put on clothes, though, and the sun setting red over the edge of the horizon is practically kissing him.

The way Dean’s husband is lazing around on the wooden planks, lying on his side on the dry portion of their boat, is really distracting in a completely different way from when he’s got his sea arms out and flashing proud colors. His now-human thighs are on full luscious display. The curve of his hip just by _itself_ is a poem Dean wants to have in his mouth over and over again.

“How d’you expect me to eat with you looking like that?” Dean grumbles, after the third time he almost stabs himself with a fork because he can’t stop staring.

Cas chuckles and rolls onto his back, patting his belly—a little softer and rounder with his own recent meal underwater. He’s half-hard, thick and tempting against his leg, and that’s _really_ not helping. “You see?” he teases. “I think the same about you. That’s why I already ate.”

Married life is totally making Dean a sap. Yep.

If he had any shits to give about that, he might even be bothered by it.

Dean rinses in fresh water before bed—Cas doesn’t, and neither of them have any explanation for why salt doesn’t crust on his skin, but it doesn’t. They don’t bother with the lamp, and the cabin’s tiny little window shows the sky turning the same deep, deep blue as Cas’s sea-arms, then grey, then nothingness. Dean thinks about crawling down Cas’s body again—maybe sucking him off low and slow, giving him a nice little reminder of why it’s fun to be in _this_ form, too—but the long shadow next to him that is Cas is close enough that even in the growing dark, he’s looking content and sleepy. Though that may be because the edges of his irises are, very faintly, glowing with happiness.

Dean jokes about Cas being boneless in his true form, sometimes, but maybe there’s something to it, because he’s never seen anyone go _relaxed_ the way Cas does.

They’ve had plenty of sex today. It’s not that they _can’t_ have more, but there’s tomorrow. The day after. There’s no rush. Dean nuzzles under Cas’s jaw, tucking his nose under his chin to give him a gentle bite just where his pulse beats, and Cas sighs, huge and soft.

There’s no rules about not going on honeymoons again, right? There better not be. They’re definitely doing this again.

“Dean?” Cas murmurs.

“Mm?”

“If Benjamin can help us, I will be very happy,” he says, soft and low, and he wraps himself greedily around Dean’s side with one arm and one leg in a way that sometimes makes Dean tease him about wishing he had more limbs again. “I would very much like to see your sea-arms, your colors.” His eyes are very close, and even in the dark his expression is so, so damned earnest. “But I think it is important that you know that I _love_ your body just as it is, and I will not be at all disappointed if there’s nothing that Benjamin or _anyone_ can do.”

Ah, fuck, _fuck,_ Cas really needs to warn him before he flips Dean’s chest like that. Loving him this much _hurts_ sometimes. Okay, so Dean’s not the only sap between the two of them.

They’re kind of busy kissing for a while, after that.

“So… are you some big hero under the water that I don’t know about?” Dean teases, gently, reshuffling Cas’s arm so it’s not pressing down on his chest. “’Cause you _say_ that your people think you’re weird, but Benjamin seems to like you just fine.”

“Oh…” Cas’s fingers laze gently up Dean’s side before they still. He murmurs, “No, nothing like that. I didn’t like other cecaelia in my territory, but I didn’t mind sharing with a pod of mermaids. Benjamin’s calf got herself stuck in a deep sea-cave.”

“Yeah?” Dean murmurs, sleepy. “Calf, huh? Mermaid baby?” He really should be writing some of this stuff down for Sam, since Cas mostly won’t talk about it with Dean’s little brother. But it’s too dark, and he really doesn’t care enough right now. Sorry, Sammy.

“Yes, mermaids are closer to humans in that way, they have liveborn young.” Castiel wrinkles his nose, playfully; Dean can feel it against his cheek. “I’m not sure why anyone would develop like that, though. I suppose some fish do it, too, and all the water mammals. It all seems very uncomfortable.”

Dean doesn’t think that eggs seem any more comfortable—it sort of seems like every method of producing small people has its ups and downs—but what does he know? “So how’d you get her out?”

“I didn’t do much. But my sea-arms fit into the cave’s opening where Benjamin’s wouldn’t, that’s all, so I could grab her. Silly thing.” Cas shrugs. “I’m sure she would have come out on her own, eventually—but, well, there were tiger sharks in the water that we hadn’t succeeded in running off yet. They’re very rude animals, _very_ stupid, and they’ll try to take hatchlings or calves sometimes. Not like greys or reefs at _all_.”

Dean hums, thoughtfully. “Damn. Well, no wonder she’s willing to help us, if you saved her kid.” Dean only has a few memories of his and Sam’s mom; he doesn’t remember the night she died saving them from a fire at all, and sometimes, he’s grateful for that. But the thought still makes his throat stick a little, and he clears it. “Moms are like that, I hear.”

“Hm? Oh. Well, I suppose. Though Benjamin is male. Didn’t you see his ventral fins?”

Dean frowns, but that statement still takes a second to register before he blinks himself all the way back awake. “What?”

Cas cocks his head against Dean’s temple. “Benjamin is a male,” he says, slowly, like he’s not sure why he has to repeat himself. “So, well, yes, he’s grateful, but he is not anyone’s mother. His mate was human, so I thought he’d be sympathetic to us? She died a very long time ago, though. A hundred years, perhaps?”

Dean’s pretty sure it’s not the lateness of the hour that’s making him not understand things, here. ‘Cause he’s _really_ wide awake, now. “But… wait, but… she’s—I mean, _he’s_ got…”

Yeah, that’s him, Dean Winchester, the very fucking soul of eloquence.

“Oh, are you referring to his mammary structures?” Cas yawns, and his voice is trying to slide down into a mumble. He sighs and snuggles closer. “Yes, both males and females have them. Mermaids aren’t sexually dimorphic the way humans and cecaelias are.”

Cas says these things like he really has no idea why Dean doesn’t know this. This is probably how Cas felt when Dean told him that yeah, caterpillars and butterflies _are_ the same animal.

“So… he’s a merman, then?” Dean blurts.

Sometimes the weirdness of what comes out of Dean’s own mouth these days shocks him sometimes.

But who can blame him? Right now it’s the _hundred years_ that’s really fucking with his head, because Dean’s never once in the entire time they’ve known each other asked how _old_ his husband is. It’s never seemed to matter, because Cas looks like he's floating somewhere around Dean’s age anyway—ha ha, floating, _ugh_ —and with his curiosity and his quiet joyfulness and all, he sometimes seems _younger._ Dean wasn’t, until this very moment, sure that cecaelia even kept track of that kind of thing.

Cas frowns. “No, Dean,” he says, sternly, and he seems to shake himself a little more awake. “Benjamin’s not a merman any more than I’m a cecaelio, there’s no such thing. He’s a male mermaid. Please don’t say things like that in front of him, you’ll offend him.”

Cas doesn’t ask why Dean’s shoulders are shaking with laughter at that, but the big, exasperated breath he puffs into Dean’s hair says “Oh, my mate is ridiculous and _strange_ ” louder than Cas has ever needed to say it aloud.

Yeah… okay, so maybe Dean’s still got a lot to learn.

~fin~

**Author's Note:**

> So my INTENTION was underwater hijinks and maybe them figuring out siphon sex, but then my brain decided that there needed to be a little bit of playful, silly *world-building* first. (What? Really, brain? Really?!) Nonetheless, I hope you enjoyed it! I've kind of really grown to love these two.
> 
> We're hitting the home stretch, friends! THANK YOU to everyone who's been following along this crazy Suptober journey with me... we're going to make it to 31, I think!


End file.
